The invention relates to a heating apparatus for cooking foods by means of at least one radiation source which is directed at a cook site for accommodating uncooked food or a container for food to be cooked, and which emits optical radiation with a visible content and an infrared content. Between the radiation source and the cook site a filter is disposed having a support body made of a material substantially transparent to all the radiation, which is provided with an interference layer system which reflects the visible content of the radiation at least predominantly back to the radiation source and passes the infrared content of the radiation at least predominantly to the cook site.
A heating apparatus of this kind, configured as a cook plate, is state of the art. In this apparatus what is involved is heating a cooker (cooking pot) placed on the cook plate with the infrared content of the optical radiation without letting the intensely visible part of the optical radiation penetrate into the kitchen space. The boundary here in question, between the visible and the infrared content of the optical radiation is at about 750 nm. The support body and interference layer system form a filter with a spectral transmission characteristic whose center lies at the above mentioned 750 nm.
Interference layer systems are characterized by alternating sequences of alternating layers of high refraction and low refraction. In the case of the system pertaining to the state of the art, titanium dioxide with an index of refraction of about 2.25 and silicon dioxide layers with an index of refraction of about 1.45 are used as the layer materials. In order then to obtain a sufficient division by the filter of a corresponding spectral transmission characteristic, it is necessary to apply a total of 43 alternating layers of the materials mentioned. This is a comparatively very difficult process which has to be performed in a vacuum, preferably by depositing the said layer materials by electron beam vapor deposition. Manufacture, therefore, is accordingly expensive.
The smaller the number of layer is, the better is their tension free build up and the firm bonding of these layers to the support body, the so called substrate, and to one another.